Cytoplasmic male sterility (CMS) is a maternally inherited inability to produce functional pollen and has been successfully used in commercial production of hybrid seed, avoiding the drawbacks of hand or mechanical emasculation (Kaul, 1988). Breeders produce hybrid seed using a CMS system by developing female lines that carry CMS cytoplasm but lack restorer genes and by developing male lines that carry the appropriate restorer genes. F1 hybrid seed produced by the female lines carry the CMS cytoplasm but yield fertile plants because of the action of the paternally contributed nuclear restorer genes.
More than 40 sources of CMS have been found in maize and were classified into three major groups by differential fertility restoration reactions. These groups are designated as CMS-T (Texas), CMS-S (USDA) and CMS-C (Charrua) (Beckett 1971). In the CMS-T group, two dominant genes, Rf1 and Rf2, located on chromosomes 3 and 9, respectively, are required for the restoration of pollen fertility (Duvick 1965). The S-cytoplasm is restored by a single gene, Rf3 on chromosome 2 (Laughnan and Gabay 1978).
Rf genes have been cloned or mapped to high resolutions from several plant species, for example, Rf2 from maize (Zea mays) (Cui et al., 1996), Rf-PPR592 from Petunia (Petunia hybrida) (Bentolila et al., 2002), Rfo from radish (Raphanus sativus) (Brown et al., 2003; Desloire et al., 2003; Koizuka et al., 2003), Rf1 and Rf2 from sorghum (Sorghum bicolor) (Klein et al., 2005), Rf1a and Rf1b from rice (Oryza sativa) for BT-type CMS (Kazama and Toriyama, 2003; Akagi et al., 2004; Komori et al., 2004; Wang et al., 2006), Rf17 (RMS) from rice (Oryza sativa) for CW-type CMS (Fujii & Toriyama, 2009), Rf1& Rf2 from monkey flower (Mimulus guttatus) (Barr & Fishman, 2010). Rf4 for CMS C-type of maize was recently cloned.